Because the quality of the LED backlight affects the stability, luminosity and color of images, a LED backlight is one of the important components of the LCDs. In the current design of backlights, LEDs replace the Cold Cathode Fluorescent Lamp (CCFL) technology rapidly because LEDs have the advantages of less requirement of voltage, ease of use, stronger dimming capacity, no mercury and higher efficiency. With the continuous improvement of the LEDs in the aspects of luminosity and cost, the LEDs are gradually applied to large LCDs.
Presently the dimming method for a LED backlight is PWM (Pulse Width Modulation) dimming. The value of a LED current is at its maximum in the PWM positive duty cycle, and the average value of a LED current is regulated by change of the positive duty cycle, so that the LED backlight is dimmed. An example is as below.
Suppose: LED Vf=3.3V, and If=60 mA, and dimming frequency f=100 Hz, and dimming duty cycle D=50%. If the light-emitting luminance of the LED is measured by the average power, the power corresponding to the maximum luminance is w=Vf*If*1/f=3.3×0.06×1/100=1.98 mW. When the dimming duty cycle D=50%, the power corresponding to the light-emitting luminance is w=3.3×0.06×1/100×1/2=0.99 mW. It is therefore obvious that the duty cycle D and the light-emitting power are correlated.
The PWM dimming frequency is generally between 100˜500 Hz, within the hearing frequency range of a person. Because mechanical vibrations can be generated by the coils of an inductor when a low frequency switching signal acts on a wound inductor, the frequency of the mechanical vibration just falls within the above frequency; and the noise emitted by the inductor can be heard by the ears of a person. Meanwhile, because the fixed delay in the PWM dimming consumes a part of the fixed cycle, the dimming uniformity is reduced.